Photon counting apparatus

ABSTRACT

An apparatus for photon counting is disclosed that comprises a sensor element ( 5 ) comprising a photon detector ( 8 ) and a capacitance ( 2 ) for applying a potential difference across the detector ( 8 ). The apparatus also comprises switching circuitry ( 6 ) for connecting the sensor element ( 5 ) operatively to a voltage source ( 4 ) during a refresh period to charge the capacitance ( 2 ) to achieve a predetermined potential difference across the detector ( 8 ) above a threshold value, and for substantially isolating the sensor element ( 5 ) from the voltage source ( 4 ) during a sensing period in which the charged capacitance ( 2 ) maintains a potential difference across the detector ( 8 ) above the threshold value and in which the detection of a photon by the detector ( 8 ) causes at least some of the charge on the capacitance ( 2 ) to discharge through the detector ( 8 ). The apparatus also comprises control circuitry ( 12 ) for controlling the switching circuitry ( 6 ) to initiate refresh and sensing periods in alternating sequence. In one aspect, the control circuitry ( 12 ) is adapted to control the switching circuitry ( 6 ) to initiate successive refresh periods at predetermined intervals.

The present invention relates to a photon counting apparatus.

Geiger-mode avalanche photodiodes are intended for operation above thediode's breakdown voltage, with the large electric field inherent inoperating the diode above the breakdown voltage allowing the diode to besensitive to a single photon of light incident upon the detector. Asingle photon entering into the detector produces an electron hole pair,and the pn diode then separates the charge carriers (electrons orholes), amplifying the separated carrier. By setting the operatingvoltage above the breakdown voltage, a self-sustaining avalanche ispossible. In typical operation, this self-sustaining avalanche must belowered through passive or active quenching circuits to below thebreakdown voltage. Known devices are generally required to quench aphoton counting detector in the fastest time possible.

U.S. Pat. No. 6,541,752 “Monolithic Circuit of Active Quenching andActive Reset for Avalanche Photodiodes” discloses the monolithicintegration of transistors with nmos and pmos transistors. With thisapproach, the quench circuit relies on the fast detection of thebreakdown event by the integrated quench circuit. In one example, thedetector is allowed to break down until the circuitry is able to reactto the increase in potential/charge at the sense node and produce asuitable voltage pulse to actively lower the voltage on the diode tobelow the breakdown voltage. The device must then actively or passivelyreset the voltage on the detector to above the breakdown voltage so thata new photon can be detected.

U.S. Pat. No. 4,945,227 “Avalanche Photodiode Quenching Circuit”discloses a sensor device in which the diode is biased through a lowresistance series with the diode for recharging the photodiode. Acomparator is buffered from the diode through a Schottky diode. During abreakdown event, the comparator must react very quickly to increases inthe voltage at the sense node to lower the bias on the diode.

U.S. Pat. No. 5,194,727 “Avalanche Photodiode Quenching Circuit withResetting Means Having a Second Amplifier” discloses circuit amplifiersto provide latching of a comparator which monitors for the breakdown ofthe avalanche photodiode. This circuit is set up for high speedoperation, ˜40 MHz.

U.S. Pat. No. 5,168,154 “Electrical Avalanche Photodiode QuenchingCircuit” discloses the use of a current source to bias a photodiode. Twoinputs are provided to the comparator which are fed by the currentsource. During breakdown, the inputs to the comparator are altered bythe current flowing through the circuit. This variation causes theoutput of the comparator to switch state and thus quench the detector.

U.S. Pat. No. 5,532,474 “Active quench circuit and reset circuit foravalanche photodiode” discloses a special inductor-based circuit toprovide a negative voltage based quench platform. The inductor is usedto prevent the flow of current to the positive power supply and to helpin resetting the APD node as quickly as possible.

The paper “Geiger-Mode Avalanche Photodiodes for Three-DimensionalImaging”, Brian F. Aull, et. al., Lincoln Laboratory Journal, vol. 13,no. 2, 2002, describes a manner in which a transistor switch is used tobias the capacitance of a photon counting diode. The transistor switchis closed to allow the diode to charge to its full potential and then itis open circuited to allow the device to detect a photon event. Thecircuitry then monitors the state of the diode for any changes in thevoltage on it. When the changes occur, the event is immediatelytime-stamped and recorded to the exterior.

Known circuitry relies on the full avalanche of the detector during abreakdown event. It is required that the device fully breaks down andthat a current of significant magnitude flows for a duration long enoughthat the output can be sensed by a comparator or other voltage/currentsensing electronics. This means that a large current must flow throughthe detector during a breakdown event, which in turn increases theafter-pulsing and increases optical coupling. After-pulsing is caused bytrapped charge during breakdown events being released after the devicehas been reset and is operational again. Optical coupling is caused bythe flow of current through a detector which is measured at ˜2E-5photons generated for every electron which passes through the diode'sdepletion region.

According to the present invention there is provided apparatuscomprising: a sensor element comprising a photon detector and acapacitance for applying a potential difference across the detector;switching circuitry for connecting the sensor element operatively to avoltage source during a refresh period to charge the capacitance toachieve a predetermined potential difference across the detector above athreshold value, and for substantially isolating the sensor element fromthe voltage source during a sensing period in which the chargedcapacitance maintains a potential difference across the detector abovethe threshold value and in which the detection of a photon by thedetector causes at least some of the charge on the capacitance todischarge through the detector; and control circuitry for controllingthe switching circuitry to initiate refresh and sensing periods inalternating sequence, wherein the control circuitry is adapted tocontrol the switching circuitry to initiate successive refresh periodsat predetermined regular intervals.

The detector may comprise an avalanche photodiode, and the potentialdifference may be a reverse bias potential difference. The thresholdvalue may be that required to achieve Geiger mode operation of thephotodiode. The threshold value may be the breakdown voltage of thephotodiode.

The control circuitry may be adapted to control the switching circuitryin dependence on a timing signal. The timing signal may be a clocksignal.

The control circuitry may be adapted to control the switching circuitryto initiate successive sensing periods at predetermined regularintervals.

The control circuitry may be adapted to interrogate the sensor elementat predetermined regular intervals to determine whether a discharge hasoccurred.

The control circuitry may be adapted to interrogate the sensor elementbefore a refresh period is initiated.

The control circuitry may be adapted to interrogate the sensor elementduring a refresh period.

The control circuitry may be adapted to control the switching circuitryin dependence upon a property of the sensor element. The controlcircuitry may be adapted to determine from the property whether adischarge has occurred. The control circuitry may be adapted todetermine the timing of the discharge from the property. The propertymay be the potential difference across the sensor element.

The control circuitry may be adapted to initiate a refresh period inresponse to the detection of a discharge.

The control circuitry may comprise sensing circuitry adapted to output asignal relating to the potential difference across the sensor element.

The signal may indicate whether the potential difference has droppedbelow a first predetermined level during a sensing period.

The signal may indicate that a discharge has occurred when the potentialdifference has so dropped.

The control circuitry may be adapted to initiate a refresh period inresponse to the signal indicating that the potential difference has sodropped.

The first predetermined level may be less than or equal to the thresholdvalue.

The signal may also indicate when the potential difference has sodropped, for providing timing information relating to this event.

The control circuitry may be adapted to initiate a sensing period inresponse to the signal indicating that the potential difference hasrisen above a second predetermined level during a refresh period.

The second predetermined level may be greater than or equal to thethreshold value.

The signal may also indicate when the potential difference has so risen,for providing timing information relating to this event.

The sensing circuitry may comprise a comparator having one input forreceiving a reference voltage for setting the predetermined level andanother input for receiving a voltage dependent on the potentialdifference across the sensor element.

The apparatus may comprise sensing circuitry adapted to sense whether adischarge has occurred.

The sensing circuitry may be adapted to sense whether a dischargeoccurred in a particular sensing period by determining the amount ofcharge required to recharge the sensor element during the subsequentrefresh period.

The sensing circuitry may be adapted to sense whether a dischargeoccurred in a particular sensing period by monitoring the voltage acrossthe sensor element during the subsequent refresh period.

The apparatus may comprise photon counting circuitry adapted to update aphoton counter in response to the detection of a discharge. The photoncounting circuitry may update the photon counter with reference to adark count or similar reference for the sensor element or apparatus.

The capacitance may comprise a capacitor connected in parallel with thedetector.

The capacitance may comprise the parasitic capacitance of the detector.

The capacitance may comprise only the parasitic capacitance of thedetector.

The apparatus may comprise a plurality of sensor elements arranged as anarray of cells.

The array may be a two-dimensional array.

Each cell of the array may have dedicated switching circuitry. Each cellof the array may have dedicated control circuitry. Each cell of thearray may have a dedicated voltage source. Each cell of the array mayhave dedicated sensing circuitry.

The control circuitry may be operable to control the switching circuitryfor a plurality of cells of the array in sequence.

Each cell of the array may have dedicated photon counting circuitryadapted to update a per-cell photon counter in response to the detectionof a discharge. The apparatus may comprise circuitry for reading theper-cell photon counters at predetermined regular intervals.

It should be noted that another aspect of the present invention is basedon the first aspect of the present invention, having such dedicatedphoton counting circuitry as mentioned above, but wherein the controlcircuitry is not necessarily adapted to control the switching circuitryto initiate successive refresh periods at predetermined regularintervals, but may be so adapted.

Each cell of the array may have dedicated circuitry adapted to determinethe timing of a discharge.

The control circuitry may be operable selectively to operate only one ormore subsets of the cells at a time.

The apparatus may be operable in a mode in which the cells are arrangedinto a plurality of groups, each group comprising one or more cells,with information regarding light intensity being provided on a per-groupbasis.

The predetermined potential difference may be within 50% of thethreshold value. The predetermined potential difference may be within25% of the threshold value. The predetermined potential difference maybe within 5% of the threshold value. The predetermined potentialdifference may be within 2% of the threshold value.

The predetermined potential difference may exceed the threshold value byan amount allowing a predetermined length of sensing period before arefresh period is required to recharge the sensor element.

The detection of a photon by the detector may cause the potentialdifference across the detector to drop substantially to zero.

Therefore, compared with known architectures, an embodiment of thepresent invention enables a Geiger-mode avalanche photodiode to provideinternal quenching without the use of external passive or activequenching circuitry.

An embodiment of the present invention has the ability to quench anddetect a photon arrival at the detector with a means of self-limitingthe current flowing through the detector. This allows a detector, orarray of detectors, to be biased in Geiger-mode, using internal orexternal capacitance to maintain the electric field of the diode. Amajor advantage with this approach is that the structure itself iscapable of self-limiting the current flow through the detector. Currentflow during the breakdown event can be minimised to the theoreticallylowest level, which will limit the optical cross-talk and after-pulsingwhich degrade current device operation and affect all currentstate-of-the-art technology. An embodiment of the present invention canbe applied to photon-counting single detectors, one-dimensional arraysof detectors, and two-dimensional imaging arrays.

Reference will now be made, by way of example, to the accompanyingdrawings, in which:

FIG. 1 is an illustrative circuit diagram showing a first embodiment ofthe present invention;

FIG. 2 is a block diagram showing an array implementation embodying thepresent invention;

FIG. 3 is an illustrative diagram showing one physical implementation ofan embodiment of the present invention;

FIG. 4 is an illustrative circuit diagram for use in explaining a secondembodiment of the present invention;

FIG. 5 is an illustrative circuit diagram for use in explaining a secondembodiment of the present invention; and

FIGS. 6 to 10 are further diagrams illustrating embodiments of thepresent invention or aspects thereof.

FIG. 1 is an illustrative circuit diagram showing an apparatus 1according to a first embodiment of the present invention. The apparatus1 comprises an array 7 of cells 10 in communication with controlcircuitry 12. In this embodiment, each cell 10 of the array 7 has thesame construction, and accordingly only one of the cells 10 will bereferenced with numerals and described herein.

The cell 10 comprises a sensor element 5, itself comprising an avalanchephotodiode (APD) photon detector 8 and a capacitance 2 for biasing(applying a potential difference across) the APD 8.

The cell 10 also comprises bias circuitry 4 for supplying a voltagesource to the sensor element 5, and switching circuitry 6 disposedbetween the bias circuitry (voltage source) 4 and the sensor element 5.

During a refresh period, the switching circuitry 6 is actuated toconnect the sensor element 5 to the voltage source 4. This charges thebias capacitance 2 to achieve a reverse-bias potential difference acrossthe APD 8 that is greater than the breakdown voltage of the APD 8, so asto enable Geiger mode operation of the APD 8 during a subsequent sensingperiod.

During a sensing period following after the refresh period, theswitching circuitry 6 is actuated to substantially isolate the sensorelement 5 from the voltage source 4. During the sensing period, thecharged capacitance 2 maintains a reverse-bias potential differenceacross the APD 8 greater than its breakdown voltage. The detection of aphoton by the APD 8 during the sensing period causes the charge on thecapacitance 2 to discharge through the APD 8.

The switching circuitry 6 is controlled in this manner by the controlcircuitry 12 to initiate refresh and sensing periods in alternatingsequence. A refresh period can be initiated before a sensing period hasended naturally through a breakdown event or natural discharge. Therefresh and sensing periods preferably follow one after the other inquick succession without gaps, but this is not essential; otheroperations, for example control operations, may be interspersed with thealternating sequence of refresh and sensing periods, and there may begaps between refresh and sensing periods.

In the first embodiment, the apparatus 1 is operated in a clockedmanner, with the control circuitry 12 controlling the switchingcircuitry 6 to initiate successive refresh periods at predeterminedintervals. The control circuitry 12 also controls the switchingcircuitry 6 to initiate successive sensing periods at predeterminedintervals. The timing of the refresh and sensing periods is controlledby a system clock. The control circuitry 12 is also adapted tointerrogate the sensor element at predetermined intervals to determinewhether a discharge has occurred. This interrogation can take placebefore a refresh period is initiated, or during a refresh period.

FIG. 2 is a block diagram illustrating schematically the use of aplurality of cells 10 arranged in a two-dimensional sensor array,surrounded by control circuitry. Address and control IO occurs through afinite state machine controller 14. A column decoder 15 and row decoders15 decode the address so as to enable random access to any one of thecells in the array using the preset controllers and read buffers.Interrogated information from the accessed cell is stored in the databuffer 18 before passing out on the data I/O line. This is akin torandom access memory, with each cell in an embodiment of the presentinvention storing one bit of information. The content of the celldepends on recent event history, with an event being triggered by aphoton hitting the active area of the APD. FIG. 3 shows one possiblephysical implementation of such a device, in which a sensor array isflip-chipped onto an ASIC (Application Specific Integrated Circuit).Field Programmable Gate Array implementation (FPGA) is also possible.

Therefore, as explained above, an embodiment of the present inventionuses a capacitance 2 to hold the diode 8 at a voltage above thebreakdown voltage. When the detector 8 turns on because of a photonarrival, the voltage on the holding capacitor 2 and on the APD 8 islowered to the breakdown voltage or below. This effectively ends thesensing period because the APD 8 is no longer reverse biased to asufficient extent to enable Geiger mode operation. The sensing periodcan also be ended by thermally-generated carriers or by the naturaldischarge of the capacitance 2 over time.

It will be appreciated that, although the bias capacitance 2 is shownand described as being provided by a capacitor element, it is also thecase that in many applications the internal parasitic capacitance of theAPD 8 itself will be sufficient to provide the required capacitance 2,without the need for any external capacitor element such as thatdepicted in FIG. 1. In any case, even with an external capacitorelement, the parasitic capacitance of the APD 8 will contribute to theoverall bias capacitance 2.

A major difference between an embodiment of the present invention andthe prior art is that, in an embodiment of the present invention, theonly current that flows through the detector 8 during the breakdownevent is that which is stored on the APD parasitic and/or externalcapacitance 2 required to hold the voltage to a suitable level. Thispresents a new device architecture for photon counting suitable for bothsingle and highly dense arrays of detectors.

An embodiment of the present invention provides a means of setting thevoltage on the APD 8 and then leaving the APD 8 for a certain fixed orvariable period of time, and a means to detect that a breakdown hasoccurred and recharge the APD 8 to the required voltage.

An embodiment of the present invention provides a device architecturethat allows for photon counting to occur without the need for passive oractive quenching circuitry to reset the detector 8 during an avalanchebreakdown event. The device architecture operates so that a voltage isapplied to the detector 8 and maintained by the internal or externalcapacitance 2.

The leakage current through the diode 8 is low enough (typically oforder of 100 fA) that the voltage can be maintained for a length of time(determined by the RC time constant of the system) sufficient to providea useful sensing period. Once the diode 8 is biased it can be left for apredetermined length of time, determined at least in part by thecapacitance 2.

A photon or thermally-generated electron/hole which comes into thedepletion region will cause a breakdown. During the breakdown event, thecharge on the capacitor 2 will be discharged through the diode 8 toground. Because the voltage source 4 is removed from the detector 8, theonly current that will flow during the breakdown event is the chargestored on the diode and any external capacitance 2. Once the charge isreduced to a level which results in a potential difference equal to thebreakdown voltage, the device will effectively be quenched and turnedoff, and the sensing period comes to an end.

The external/internal bias circuitry 4 is then re-applied to the diode 8to refresh the diode voltage to a level above the breakdown voltage. Bymonitoring the voltage on the diode 8 when the diode is being reset (ortopped up), it is possible to determine if a photon has been detected.

FIG. 6 shows an example of the architecture that can be used to providethe ability to switch the voltage onto the diodes and then allow thedevices to be read out in a scanned manner. The sequence for operationis as follows:

1. Start with no word lines selected (W0-W3)

2. Connect APD bit lines (B0-B3) to buffer inputs

3. Assert word line and read buffer output

4. Switch bit line to charge voltage

5. De-select the word lines before repeating the sequence

FIG. 4 is an illustrative circuit diagram for use in explaining a secondembodiment of the present invention. In second embodiment of the presentinvention, a cell 20 includes voltage/current sense capability to detectthe presence of a change in the voltage on the APD 8, caused by abreakdown, or the flow of current through the APD 8 to ground. Thecircuit then signals to the bias circuitry to recharge the APD.

Therefore, while in the first embodiment the apparatus operates only ina fully clocked mariner in which the APD 8 is interrogated at setintervals, in the second embodiment the apparatus is also provided withthe ability to operate in free-running mode in which the cell 20 can setand reset itself dependent on the photon or breakdown times, thusoperating independently.

As shown in FIG. 4, the cell 20 comprises a bias capacitance 2 and APD 8(sensor element 5), switching circuitry 6 and bias circuitry 4corresponding to those parts in the first embodiment. In the secondembodiment, however, the cell 20 also comprises dedicated controlcircuitry 22 having an nmos transistor 24 connected between ground and aresistor R, itself connected to a positive voltage source. The gate ofthe nmos transistor 24 is connected to the cathode of the APD 8, while acontrol signal to the switching circuitry 6 is taken from a point Abetween the transistor 24 and the resistor R.

In operation of the second embodiment, during a refresh period theswitching circuitry 6 is operated to connect the voltage source 4 to thecapacitance 2. The capacitance 2 charges up to a point where the mostransistor 24 turns on, after the APD 8 is sufficiently reverse biasedto enable Geiger mode operation. Point A then takes a low value, causingthe switching circuitry 6 to isolate the voltage source 4 from thesensor element 5. This commences the sensing period.

During the sensing period, if a photon causes a discharge in the APD 8,the potential difference across the APD 8 will drop to below thebreakdown voltage of the APD 8, and this in turn will cause thetransistor 24 to turn off. Point A will then take a high value, causingthe switching circuitry 6 to connect the voltage source 4 to the sensorelement 5. This again commences the refresh period, and the cyclecontinues in this way.

FIG. 5 is a circuit diagram showing a third embodiment of the presentinvention. The third embodiment is similar to the second embodiment, butis more elaborate. The construction of the third embodiment will bereadily understood from the circuit diagram shown in FIG. 5, and theoperation is as follows.

For operation a high voltage (Vhv) which is below the breakdown voltageis placed on the diodes anode or cathode, depending on the circuitconfiguration. A voltage (Vex) is then placed on the opposite terminalto bias the diode above the breakdown voltage (Vbr). The total voltageacross the diode is therefore Vhv+Vex>Vbr.

During a breakdown event, the voltage at node (a) will be reduced sothat the total voltage across the diode is less than the breakdownvoltage.

When node (a) is low, transistor N1 is turned off. This causes thevoltage on node (b) to increase towards +5V. Node (b) forms thenon-inverting input to the comparator C. When this input is above thereference voltage, Vref, the inverted output Q′ of the comparator Cswings low. When Q′ goes low, this causes the transistor N2 to turn off.This in turn causes node (c) to go high, which turns on transistor N3(corresponding to the switching circuitry of previous embodiments).

When transistor N3 is on, node (d) goes high, from the voltage sourceVAR. This voltage source VAR connects through a Schottky diode (forspeed) to the APD 8, thus restoring the APD bias voltage back to a valuegreater than the breakdown voltage of the APD 8 in preparation for thesensing period.

This causes transistor N1 to turn on, causing node (b) to go lower thanVref. The low value at the non-inverting input to the comparator Ccauses the output Q′ to go high, turning the transistor N2 on. Node (c)is caused to go low, which turns off transistor N3. This disconnects thevoltage source VAR from the APD 8, and thus begins the sensing period.

The sensing period ends, for example, when a photon causes a breakdown,and the cycle repeats as above.

Although the same core architecture is used as described for theprevious two embodiments to charge the capacitor and detect that abreakdown has occurred, in the third embodiment timing information isalso preserved.

For example, in one mode of operating in the first embodiment,individual cells in an array are operated alternately in refresh andsensing modes according to a system clock, being charged up during arefresh period, left to stand during a sensing period, and theninterrogated to determine whether a discharge occurred during thesensing period. This allows the operation of large arrays of detectorsbut the absolute photon arrive time is lost because of the serial natureof the readout. At best, the timing information would be limitingknowing that the discharge must have occurred at some time during thesensing period.

Having a separate readout circuit for each detector is limited by thesize and complexity of the required circuitry both in interrogatinglarge arrays but also in the signal and data processing required to dealwith large detector arrays and large amounts of data from those arrays.For smaller arrays, where the timing information of the photon isdesired, it is possible by using a dedicated charge and readout circuitfor each pixel, to retain photon arrive time, and this is the approachtaken with the third embodiment. Another embodiment would include theability to switch from serial output for the entire array of pixels to alimited number of pixels which operate with a very fast sense period orin a free running mode.

In the third embodiment, each cell is charged by the charge/rechargecircuitry. When a breakdown event is detected, the recharge circuitry isused to recharge the device and also to output a signal (from thecomparator C). The recharge sets the device operating again and theoutput signal signifies the arrival time of the photon.

Advantage of this over prior art circuitry are that significantly lesscurrent flows through the detector area during a breakdown, causing lesspixel to pixel coupling, less heat to be dissipated, less chance of anafter-pulse and a very short dead-time of the detector.

Known circuitry requires that the detector must first break down fullyto generate a large current flow which can be measured and detected byexternal (or monolithic) circuitry. This means that each time that aphoton is detected it is required that a relatively large current flowthrough the active area of the photon counter. An embodiment of thepresent invention instead uses internal and external capacitance toprovide bias to the detector. During a breakdown event the charge on thecapacitor is removed through the low resistance diode to ground. In oneaspect, the circuit then scans the detector again to effectivelyread/write the individual diode. This allows large arrays with verytight pixel pitch to be formed, limited only by the speed at which thedata can be read out by the external/monolithic circuitry that biasesthe diode.

As described above, an apparatus according to an embodiment of thepresent invention can be provided with cells that are operated in aclocked fashion but which also have the capability of detecting abreakdown event and initiating a refresh period in response.

It will be appreciated that various features of the above-describedembodiments can be combined. It will also be appreciated that thefunctionality of the circuitry shown and described herein can beprovided in many different ways, the particular implementations hereinbeing mere examples.

In another embodiment of the present invention, is cell of the array isprovided with its own counter in which is stored the number of times thedetector for that cell has fired. The per-cell counters can either beread at predetermined intervals, for example every 1 μs or every 10 μs,they can be read at arbitrary intervals, for example when a counter hasreached its maximum count and needs to be reset.

The per-pixel counter may have the ability to reset itself independentlyof operation from the write/read scanning mechanism. One way to achievethis is would be to provide a delay circuitry in the pixel to reset thedevice once it has broken down and had a sufficient time to remove thecharge from the pixel. Such circuitry is illustrated schematically inFIG. 7, and operates as follows:

1. APD anode is biased at negative breakdown

2. Parasitic capacitance maintains Vapd

3. Incident photon causes Vapd to drop quickly

4. Delay resets device after short time

5. Gives a pulse for every photon which can be counted with appropriatecircuitry

One possible implementation of the delay circuit is shown in FIG. 8, andthis would allow the device to be reset through an active resetmechanism on the top transistor and the bottom transistor could be usedto remove any remaining charge on the diode itself.

This is similar to an active quench circuit in which the charge on thediode is actively removed after a breakdown event. This is useful inthis instance to optimise the counting mechanism. The inverter chaincould be used to delay the reset of the diode. This circuit also has anoptional vref input which would allow the threshold of the circuit to beset to allow the fastest operation time possible. Vref does not have tobe included for it to work but it will work faster if it is included inthe circuit.

A block level diagram showing the counter, quench and buffer to theoutside are shown in FIG. 9. This would allow the device to reset itselfand keep counting after an event. The buffer could be read out at apredetermined interval or at any other time, for example when it isfull.

An embodiment of the present invention, for example according to theimplementation shown in FIG. 10, can operate in the following threedifferent modes simultaneously by the following:

1. Digital APD

-   -   1. Active quench circuit disabled    -   2. Value is read    -   3. Set pulse allows active quench    -   4. circuit to quench/reset the APD    -   5. Move on to next word line        2. Counting    -   1. Active quench circuit enabled    -   2. Timer/counter increments on events    -   3. Sample snapshots value    -   4. Scan allows transfer off chip        3. Time Stamping    -   1. Active quench circuit enabled    -   2. Timer/counter increments on clock    -   3. Timer snapshot taken with event    -   4. Scan allows transfer off chip

An embodiment of the present invention can be operated in a mode inwhich only a small subset of cells is read/write out of the entirearray. This provides an ability to window the array so as to sample onlya portion of the array at a single time. This allows a speeding up ofthe read/write refresh cycle time where that is required, so as to allowthe array to operate at higher frequencies, albeit with a lower numberof active pixels.

Timing circuitry can also be incorporated within each cell. This allowsthe device to be able to time the arrival of a photon event. The timestamp that is generated in response to a photon arriving can be read outat a later predetermined time, for example at predetermined intervals,or could be read out immediately.

The windowing ability described above allows the arrival time of aphoton to be recorded more accurately. This provides the ability to tooperate individual pixels at a higher refresh rate and to allowprecision timing of the changes in the individual pixels.

For example, one method of use would first operate the apparatus in ascanning mode across the entire array. Once a particular region ofinterest is determined, a subset of the array can be selected to allowscanning of that region of interest at a higher rate. This allow thedetermination of photon arrival times in the smaller subset of the arraywith a higher level of precision.

It is also possible to provide an apparatus that can operate in a modein which there is a reduced number of outputs, for example to provideinformation regarding the light intensity across on the entire array, orregions thereof. This would provide the ability to provide a singleoutput in which the output signal represents the signal level which isincident on the entire array; or a number of pixels can be selected toprovide a single output for those pixels only. In this way, the arraycan be segmented, with an output being provided for each segment. Eachsegmented output would then provide information regarding the lightintensity on that segment.

The output of the system can either be a digital signal or analoguesignal. With segmented outputs, this would enable the ability to providea single analogue output to the outside, or several analogue outputs.

An embodiment of the present invention can be applied, for example, invery low light level imaging applications, Laser Radar/LIDAR/RangeFinding applications, TOF applications, and low light imagingapplications requiring excellent timing resolution.

1. Apparatus comprising: a sensor element comprising a photon detectorand a capacitance for applying a potential difference across thedetector; switching circuitry for connecting the sensor elementoperatively to a voltage source during a refresh period to charge thecapacitance to achieve a predetermined potential difference across thedetector above a threshold value, and for substantially isolating thesensor element from the voltage source during a sensing period in whichthe charged capacitance maintains a potential difference across thedetector above the threshold value and in which the detection of aphoton by the detector causes at least some of the charge on thecapacitance to discharge through the detector; and control circuitry forcontrolling the switching circuitry to initiate refresh and sensingperiods in alternating sequence, wherein the control circuitry isadapted to control the switching circuitry to initiate successiverefresh periods at predetermined regular intervals.
 2. Apparatus asclaimed in claim 1, wherein the detector comprises an avalanchephotodiode, and wherein the potential difference is a reverse biaspotential difference.
 3. Apparatus as claimed in claim 2, wherein thethreshold value is that required to achieve Geiger mode operation of thephotodiode.
 4. Apparatus as claimed in claim 3, wherein the thresholdvalue is the breakdown voltage of the photodiode.
 5. Apparatus asclaimed in claim 1, wherein the control circuitry is adapted to controlthe switching circuitry in dependence on a timing signal.
 6. Apparatusas claimed in claim 5, wherein the timing signal is a clock signal. 7.Apparatus as claimed in claim 1, wherein the control circuitry isadapted to control the switching circuitry to initiate successivesensing periods at predetermined regular intervals.
 8. Apparatus asclaimed in claim 1, wherein the control circuitry is adapted tointerrogate the sensor element at predetermined regular intervals todetermine whether a discharge has occurred.
 9. Apparatus as claimed inclaim 8, wherein the control circuitry is adapted to interrogate thesensor element before a refresh period is initiated.
 10. Apparatus asclaimed in claim 8, wherein the control circuitry is adapted tointerrogate the sensor element during a refresh period.
 11. Apparatus asclaimed in claim 1, wherein the control circuitry is adapted to controlthe switching circuitry in dependence upon a property of the sensorelement.
 12. Apparatus as claimed in claim 11, wherein the controlcircuitry is adapted to determine from the property whether a dischargehas occurred.
 13. Apparatus as claimed in claim 12, wherein the controlcircuitry is adapted to determine the timing of the discharge from theproperty.
 14. Apparatus as claimed in claim 11, wherein the property isthe potential difference across the sensor element.
 15. Apparatus asclaimed in claim 1, wherein the control circuitry is adapted to initiatea refresh period in response to the detection of a discharge. 16.Apparatus as claimed in claim 1, wherein the control circuitry comprisessensing circuitry adapted to output a signal relating to the potentialdifference across the sensor element.
 17. Apparatus as claimed in claim16, wherein the signal indicates whether the potential difference hasdropped below a first predetermined level during a sensing period. 18.Apparatus as claimed in claim 17, wherein the signal indicates that adischarge has occurred when the potential difference has so dropped. 19.Apparatus as claimed in claim 17, wherein the control circuitry isadapted to initiate a refresh period in response to the signalindicating that the potential difference has so dropped.
 20. Apparatusas claimed in claim 17, wherein the first predetermined level is lessthan or equal to the threshold value.
 21. Apparatus as claimed in claim17, wherein the signal also indicates when the potential difference hasso dropped, for providing timing information relating to this event. 22.Apparatus as claimed in claim 17, wherein the sensing circuitrycomprises a comparator having one input for receiving a referencevoltage for setting the predetermined level and another input forreceiving a voltage dependent on the potential difference across thesensor element.
 23. Apparatus as claimed in claim 16, wherein thecontrol circuitry is adapted to initiate a sensing period in response tothe signal indicating that the potential difference has risen above asecond predetermined level during a refresh period.
 24. Apparatus asclaimed in claim 23, wherein the second predetermined level is greaterthan or equal to the threshold value.
 25. Apparatus as claimed in claim23, wherein the signal also indicates when the potential difference hasso risen, for providing timing information relating to this event. 26.Apparatus as claimed in claim 1, comprising sensing circuitry adapted tosense whether a discharge has occurred.
 27. Apparatus as claimed inclaim 26, wherein the sensing circuitry is adapted to sense whether adischarge occurred in a particular sensing period by determining theamount of charge required to recharge the sensor element during thesubsequent refresh period.
 28. Apparatus as claimed in claim 26, whereinthe sensing circuitry is adapted to sense whether a discharge occurredin a particular sensing period by monitoring the voltage across thesensor element during the subsequent refresh period.
 29. Apparatus asclaimed in claim 1, comprising photon counting circuitry adapted toupdate a photon counter in response to the detection of a discharge. 30.Apparatus as claimed in claim 29, wherein the photon counting circuitryupdates the photon counter with reference to a dark count or similarreference for the sensor element or apparatus.
 31. Apparatus as claimedin claim 1, wherein the capacitance comprises a capacitor connected inparallel with the detector.
 32. Apparatus as claimed in claim 1, whereinthe capacitance comprises the parasitic capacitance of the detector. 33.Apparatus as claimed in claim 32, wherein the capacitance comprises onlythe parasitic capacitance of the detector.
 34. Apparatus as claimed inclaim 1, comprising a plurality of sensor elements arranged as an arrayof cells.
 35. Apparatus as claimed in claim 34, wherein the array is atwo-dimensional array.
 36. Apparatus as claimed in claim 34, whereineach cell of the array has dedicated switching circuitry.
 37. Apparatusas claimed in claim 34, wherein each cell of the array has dedicatedsensing circuitry.
 38. Apparatus as claimed in claim 34, wherein eachcell of the array has a dedicated voltage source.
 39. Apparatus asclaimed in claim 34, wherein each cell of the array has dedicatedcontrol circuitry.
 40. Apparatus as claimed in claim 34, wherein thecontrol circuitry is operable to control the switching circuitry for aplurality of cells of the array in sequence.
 41. Apparatus as claimed inclaim 34, wherein each cell of the array has dedicated photon countingcircuitry adapted to update a per-cell photon counter in response to thedetection of a discharge.
 42. Apparatus as claimed in claim 41,comprising circuitry for reading the per-cell photon counters atpredetermined regular intervals.
 43. Apparatus as claimed in claim 41,wherein the control circuitry is not necessarily adapted to control theswitching circuitry to initiate successive refresh periods atpredetermined regular intervals.
 44. Apparatus as claimed in claim 34,wherein each cell of the array has dedicated circuitry adapted todetermine the timing of a discharge.
 45. Apparatus as claimed in claim34, wherein the control circuitry is operable selectively to operateonly one or more subsets of the cells at a time.
 46. Apparatus asclaimed in claim 34, operable in a mode in which the cells are arrangedinto a plurality of groups, each group comprising one or more cells,with information regarding light intensity being provided on a per-groupbasis.
 47. Apparatus as claimed in claim 1, wherein the predeterminedpotential difference is within 50% of the threshold value.
 48. Apparatusas claimed in claim 47, wherein the predetermined potential differenceis within 25% of the threshold value.
 49. Apparatus as claimed in claim48, wherein the predetermined potential difference is within 5% of thethreshold value.
 50. Apparatus as claimed in claim 49, wherein thepredetermined potential difference is within 2% of the threshold value.51. Apparatus as claimed in claim 1, wherein the predetermined potentialdifference exceeds the threshold value by an amount allowing apredetermined length of sensing period before a refresh period isrequired to recharge the sensor element.
 52. Apparatus as claimed inclaim 1, wherein the detection of a photon by the detector causes thepotential difference across the detector to drop substantially to zero.